Answer

In Swift, range matching is often used in switch-case for comparing values with ranges. The operators ..< (half-open range) and ... (closed range) are applied for this purpose:

let score = 76
switch score {
case 0..<60:
    print("Unsatisfactory")
case 60..<80:
    print("Satisfactory")
case 80...100:
    print("Excellent")
default:
    print("Incorrect value")
}

Features:

• Ranges must be increasing: from smaller to larger (otherwise the range will be empty).
• Matching will be successful if the value lies within the range.
• Since ranges are structures, they can be used for matching with any Comparable types.

Trick Question

What will happen if a range with an upper boundary that does not include the value (for example, 0..<5) is used in switch and the variable is equal to 5?

It is often answered that such a case will trigger, but this is incorrect: the operator ..< does not include the upper boundary. Example:

let value = 5
switch value {
case 0..<5:
    print("0-4")
case 5:
    print("five")
default:
    print("other")
}
// Output: "five"

Real Error Examples Due to Ignorance of the Topic

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Story

A developer implemented a display of user scores by ranges, using a closed range where the upper boundary was above the maximum possible value. This led to certain values falling into two intervals at once, causing the logic to work incorrectly.

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Story

In an application, a switch-case was used with ranges where one of the values did not fit into any range due to incorrect usage of the operator (for example, 0...10 and 11...20 — and the value 10.5 did not fit anywhere). The bug was discovered only in production.

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Story

In the service's code, dates were compared using range matching, using a half-open range incorrectly: the end date was not included, resulting in the exclusion of valid intervals, making it impossible to book the date on the last allowable day.